1. Long Term Evolution (LTE) System
The LTE system supports two duplex modes of Frequency Division Duplex (FDD) and Time Division Duplex (TDD), in which different types of frame structures are applied. The two types of frame structures are common in that each radio frame includes 10 sub-frames of 1 ms, where the first type of frame structure is applied to the FDD system, and the second type of frame structure is applied to the TDD system. In the first type of frame structure, 10 sub-frames in a radio frame have the same transmission direction which is in the uplink or the downlink. In the second type of radio frame, each radio frame includes three kinds of different sub-frames including a downlink sub-frame, an uplink sub-frame, and a special sub-frame, where the special sub-frame includes three components including a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS).
For a normal Cyclic Prefix (CP), a sub-frame includes 14 symbols including Orthogonal Frequency Division Multiplex (OFDM) symbols in the downlink, and Single-Carrier Frequency-Division Multiple Access (SC-FDMA) symbols in the uplink. For an extended CP, a sub-frame includes 12 symbols.
2. Spectrum Resources
Existing spectrum resources can be categorized into licensed spectrums and unlicensed spectrums, where the licensed spectrums are allocated dedicated spectrums, where different LTE operators are provided with different dedicated licensed spectrums so that the LTE operations will not suffer from interference of the other LTE operators. The unlicensed spectrums are shared spectrum resources which are not allocated, so that the resources in the unlicensed frequency bands can be shared by a number of types of systems.
At present, the spectrum resources have become increasingly insufficient as the number of mobile data services is constantly growing, so that if the networks are deployed and the services are transmitted only over the licensed spectrum resources, then the growing number of services may not be accommodated, so the services may be deployed and transmitted over the unlicensed spectrum resources in the LTE system (i.e., Unlicensed LTE (U-LTE or LTE-U)) to thereby improve the experience of a user, and extend the coverage thereof. However, there has been so far absent a definitive solution to how the LTE system operates over the unlicensed spectrum resources, and how a number of LTE systems take over the unlicensed frequency bands in a fairly and friendly manner, and also access efficiently the resources in the unlicensed frequency bands.